docs: def all_label_values(self, label_list_ids=None):
          """
          Return a set of all label-values occurring in this corpus.
  
          Args:
              label_list_ids (list): If not None, only labels from label-lists with an id contained in this list
                                     are considered.
  
          Returns:
               :class:`set`: A set of distinct label-values.
          """
          values = set()
  
          for utterance in self.utterances.values():
              values = values.union(utterance.all_label_values(label_list_ids=label_list_ids))
  
          return values